A semiconductor device such as a programmable logic device (PLD) or a central processing unit (CPU) has a variety of configurations depending on its application. The semiconductor device generally includes a memory device; the PLD includes a register and a configuration memory, and the CPU includes a register and a cache memory.
These memory devices need to operate at higher speed in writing and reading data than a main memory for which a DRAM is generally used. Thus, in many cases, a flip-flop is used as a register, and a static random access memory (SRAM) is used as a configuration memory and a cache memory.
The SRAM achieves high-speed operation with miniaturization of a transistor; however, there is a problem in that as the transistor is miniaturized, an increase in leakage current becomes obvious and thus, power consumption is increased. In order to reduce power consumption, an attempt has been made to stop the supply of power supply voltage to a semiconductor device in a period when data is not input or output, for example.
However, a flip-flop used as a register and an SRAM used as a cache memory are volatile memory devices. Therefore, in the case where the supply of power supply voltage to a semiconductor device is stopped, data which has been lost in a volatile memory device such as a register or a cache memory need to be restored after the supply of power supply voltage is restarted.
In view of this, a semiconductor device in which a nonvolatile memory device is located on the periphery of a volatile memory device has been developed. For example, Patent Document 1 discloses a technique in which data held by a flip-flop or the like is stored in a ferroelectric memory before the supply of power supply voltage is stopped, and the data stored in the ferroelectric memory is restored to the flip-flop or the like after the supply of power supply voltage is restarted.
One effective method for reducing power consumption of a semiconductor device such as a PLD or a CPU is to decrease power supply voltage. Power consumption is proportional to power supply voltage; therefore, as power supply voltage decreases, power consumption can be reduced.
However, a low power supply voltage results in a low ratio of on-state current to off-state current of a transistor. This causes a logic circuit to have an unstable output voltage and the transistor to have an increased leakage current.
In view of this, a circuit for ensuring a sufficient ratio of on-state current to off-state current of a transistor has been proposed. For example, Non-Patent Document 1 discloses a flip-flop for which a Schmitt trigger circuit is used and which can be driven at low voltage. With the use of a Schmitt trigger circuit for a flip-flop, a stable output voltage can be obtained even at a low power supply voltage, and power consumption can be reduced with a low flow-through current.